POLLUX (Beta Geminorum). Star with planet. In northern spring evenings, the "twin"
stars Castor and Pollux of the
constellation Gemini descend the
northwestern sky looking like a pair of eyes staring down at the
Earth. They are twins only in mythology, these warriors, Pollux
fathered by Zeus and divine, Castor mortal, both placed in the sky
to allow them to be together for all time. The northernmost of the
zodiacal constellations,
Gemini is also among the brightest, helped by first magnitude
Pollux and second magnitude Castor. An exception to the rule,
brighter Pollux, the sky's 17th brightest star, was given the Beta designation by Bayer, while somewhat
fainter Castor is known as Alpha Geminorum. In fact, Pollux and
Castor are nothing like twins, bright Castor a white quadruple star
with fairly class A hot components (sextuple if you count a distant
pair of companions) and Pollux an orange-colored cool (4770 Kelvin)
class K (K0) giant with a
planet, the nice
pairing with Castor making Pollux's color more vivid. From its
distance of 34 light years, we calculate a total luminosity
(incuding infrared radiation) for Pollux 46 times that of the Sun, and coupled with its temperature, a
diameter some 10 times solar, making it smaller than most of its
cool giant brethren and only a quarter the dimension of Aldebaran. Direct measures of angular
size, however, yield a somewhat smaller diameter 9 times that of
the Sun. From luminosity and temperature, the mass comes in at
around 1.8 times solar. Like so many others of the nighttime sky,
Pollux is a typical red giant that is quietly fusing helium into
carbon and oxygen in its deep core. It emits X-rays and seems to
have a hot, outer, magnetically supported corona perhaps similar to
that surrounding our Sun.

The Planet: Pollux's greatest claim to fame is an
orbiting planet, making one of the very few
giants with such companions, and the brightest planet-holding star.
With a mass at least 2.9 times that of Jupiter, the planet orbits
in a nearly circular path at a average distance of 1.69 Astronomical Units
(11 percent farther than Mars is from the Sun) with a period of 590
days (1.6 years). Like many other planet-holding stars, Pollux is
metal-rich, with an iron content (relative to hydrogen) 55 percent
higher than solar. From the planet, which is bathed with a
radiation intensity 16 times the amount we get from the Sun, Pollux
would glower in the sky with an angular diameter of nearly 3
degrees, 5.7 times bigger than we see the Sun. Not far away,
in Taurus, is
another giant with a planet, Ain
(Epsilon Tauri). (Gemini has two other
stars with orbiting planets: HR 2877 and
HD 50554.)